1. Field of the Invention
The present invention relates to a method and an apparatus for stacking sheets and, more particularly, to a method and an apparatus for stacking a pair of glass substrates constituting a liquid crystal display panel.
2. Discussion of Related Art
There has been a conspicuous rise in the popularity of a liquid crystal display device used as an image display device for a personal computer and various other monitors. In general, such a liquid crystal display device is constituted so as to make an image formed on a liquid crystal surface visible by disposing a backlight as a sheet-like light source for lighting in the backside of a liquid crystal display panel and by lighting the entire liquid crystal surface having a specified width to uniform brightness. This liquid crystal display device includes a liquid crystal display panel having liquid crystal sealed in between a pair of glass substrates.
The liquid crystal display panel is manufactured mainly by a process called a vacuum injection method. According to the vacuum injection method, a pair of glass substrates having a predetermined distance therebetween are arranged oppositely to each other, a panel including a liquid crystal inlet is dipped in liquid crystal after the execution of degassing between the glass substrates under vacuum, then the atmospheric pressure is restored and, by using a pressure difference between the inside and the outside of the panel and capillarity, the liquid crystal is injected into the panel. The panel can be obtained by applying sealant around the pair of glass substrates in a picture-frame state, and then curing the sealant. A region surrounded with the sealant of the picture-frame state becomes an image display region, and the liquid crystal is injected through the liquid crystal inlet into this image display region. The liquid crystal inlet is sealed off after the liquid crystal is injected. For a sealant material, a thermosetting or ultraviolet curable resin is used, and the sealant is cured before injection of the liquid crystal.
A problem inherent in the vacuum injection method has been long time expended for injection of the liquid crystal. In particular, there has been a problem that a great deal of time required for injection when the size of the liquid crystal display panel becomes larger.
In addition to the above-described vacuum injection method, a liquid crystal sealing-in process called a dropping method is available. According to the dropping method, sealant is coated in a picture-frame state around one of the pair of glass substrates, the other glass substrate is stacked after liquid crystal is dropped on a region surrounded with the sealant, and then the sealant is cured. Compared with the vacuum injection method, this dropping method is advantageous in that time required for liquid crystal dropping is greatly shortened. Therefore, considering manufacturing costs, the dropping method is a good manufacturing method.
Regarding the stacking of the glass substrate after the dropping of liquid crystal, the dropping method has a method executed in the atmosphere, and a method executed in a vacuum device. The method of stacking the glass substrate in the atmosphere is advantageous in that manufacturing costs can be reduced by an amount equivalent to the nonuse of the vacuum device. However, bubbles tend to remain on a stacking surface during the stacking of the glass substrate in the atmosphere. These bubbles may lead to a display failure in the liquid crystal display device. Many suggestions have been presented to solve this problem of residual bubbles. For example, Japanese Patent Laid-Open No. Hei 4 (1992)-179919 discloses a stacking method of widely spreading liquid crystal on the full surface of the glass substrate while narrowing an angle between the pair of glass substrates, liquid crystal having been coated on the corner of one glass substrate at the point of intersection where the pair of glass substrates are arranged to face each other in a surface direction in a wedge shape. Also, Japanese Patent Laid-Open No. Hei 4(1992)-179919 or Japanese Patent Laid-Open No. 2000-29051 discloses a method of widely spreading liquid crystal on a full surface while returning the glass substrate to its original planar state after the pair of glass substrates are placed to face each other, and liquid crystal is interposed while at least one glass substrate is bent in a projecting shape.
The above-described methods reduce residual bubbles. However, as long as stacking is carried out in the atmosphere, bubbles always remain to a slight extent. Especially when higher image quality is required, these methods cannot provide sufficient countermeasures.
It is therefore considered preferable to execute the stacking of the glass substrate in vacuum even in the dropping method. However, for stacking the glass substrate in vacuum, there is a problem regarding how to hold the glass substrate in vacuum. In other words, to stack the pair of glass substrates, at least one glass substrate must be aligned with the other glass substrate while it is being held. But no proper means for holding the glass substrate has been discovered yet.
As one of the most general glass substrate holding methods in the atmosphere, a vacuum chucking or holding method is available. To begin with, however, the vacuum chucking method cannot be used in vacuum, where provides no differential pressures. An electrostatic chucking or holding method using static electricity enables to hold the glass substrate in vacuum. However, in the electrostatic chucking method, it takes a long time until securely holding. In addition, an electric circuit is provided on the glass substrate of the liquid crystal display device, and there is a danger that the electrostatic chucking may cause electrostatic destruction in the circuit.
If a mechanical holding method is used, the glass substrate can be held in vacuum, holding time can be short, and the electrostatic destruction of the circuit can be prevented.
The glass substrate used for the liquid crystal display device is thin, having a thickness set equal to about 0.7 mm, and a distance between the pair of glass substrates is very small, that is, 10 micrometers or less. Further, the pair of glass substrates must be stacked in the state of highly accurate alignment. Accordingly, even if the mechanical holding method is employed, a method and an apparatus for stacking should be selected with consideration given to the above point.
The present invention was made with the foregoing problems in mind, and the object of the invention is to provide an apparatus and a method for stacking sheets accurately even in vacuum. Another object of the invention is to provide a method and an apparatus for manufacturing a liquid crystal display panel by using such stacking apparatus and method.
As one of the most general methods of mechanically holding the glass substrate, a method is available for mechanically supporting the peripheral edge of the glass substrate from a lower side. As described above, the glass substrate used for the liquid crystal display device is thin, having a thickness of about 0.7 mm. Thus, if the glass substrate is supported by its peripheral edge, then bending may occur because of the own-weight of the glass substrate.
When one of the paired glass substrates is stacked on the other with edges aligned while the peripheral edge thereof is supported by supporting means, the supporting means is held between the paired glass substrates. In the liquid crystal display device, since a distance between the paired glass substrates is small, that is, several micrometers, in order to stack one of the glass substrates on the other with edges aligned while supporting the peripheral edge thereof by the supporting means, a thickness of the supporting means must be set equal to several micrometers or less. However, it is difficult to support the glass substrate by the supporting means having a thickness of only about several micrometers.
The present invention takes advantage of the bending of the glass substrate. The bending amount of the bent glass substrate is maximum in its middle portion. In other words, by setting the middle portion as a reference for regulating a distance required by the liquid crystal display panel, the peripheral edge of the glass substrate has a distance larger than this distance. Accordingly, the thickness of the supporting means can be set larger than the distance, realizing the employment of mechanical supporting means. In this case, the position of supporting the glass substrate must be set so as to increase the amount of bending. Generally, the glass substrate used for the liquid crystal display device is rectangular. The rectangular shape has two opposing long sides and two opposing short sides. To support the glass substrate in such a way as to increase the amount of bending, the opposing short sides only need to be held. Such a holding causes bending along the longitudinal direction of the glass substrate. Otherwise, if the glass substrate is held by opposing long sides, it can make a larger distance between alignment marks on the substrate so as to achieve higher alignment accuracy.
In the liquid crystal display device, the paired glass substrates must be stacked with high accuracy. For this purpose, alignment marks are formed on the pair of glass substrates, and the glass substrates are aligned with each other while observing these alignment marks by a microscope. To execute such an alignment, the paired glass substrates must be brought close to each other by a distance (focal depth) for simultaneously focusing the alignment marks of the pair of glass substrates. Assuming that one of the glass substrates is bent, a distance between the lowest point of the glass substrate by the bending and the other glass substrate is set to be a distance within the focal depth, and an alignment mark is formed in this region. However, it is not easy to control the distance between the lowest point of the glass substrate by the bending and the other glass substrate to a distance within the focal depth. Especially when the size of the glass substrate changes to vary its bending amount, control thereof becomes much more difficult. Conversely, if the amount of bending could be kept constant irrespective of the glass substrate size, then the distance between the lowest point of the glass substrate by the bending and the other glass substrate can be easily controlled to a distance within the focal depth.
After the end of the alignment using the alignment marks, the supported state of the bent glass substrate must be released. Since the alignment has been carried out, any displacement of the each substrates after the releasing of the supported state must be avoided. When the supporting state of the supporting means supporting the peripheral edge of the glass substrate is suddenly released, almost no macroscopic displacement occurs in the position of the glass substrate because of the law of inertia. However, in the liquid crystal display device, since even microscopic fluctuation becomes a problem, with only the sudden releasing of such a supporting state, unnecessary displacement may occur in the liquid crystal display panel. To prevent such a displacement, the horizontal movement of the glass substrate supported by the supporting means must be restricted. By applying a perpendicular load on the glass substrate, the horizontal or in-plane movement of the glass substrate can be restricted. Especially, the application of this load on a position corresponding to the lowest point of the glass substrate by the bending is preferable for the prevention of the displacement.
By the way, the displacement between the laminated substrates should be less than a few micrometers, generally less than 3 micrometers, and preferably less than 1 micrometer. In order to achieve the short displacement, realignment or fine alignment may be executed after releasing the supporting state. In such case, it is preferred to execute the fine alignment in vacuum. Then, the present invention provides the method of fine alignment which can be easily performed in vacuum.
Usually, spacer is disposed on at least one substrate to control the cell gap between two substrates appropriately. As the spacer, beads can be disposed or columns (referred as post spacers) can be formed on the substrate. At the case of using the post spacers, if the tips of the columns come into contact with the surface of the substrate facing oppositely, then it generates very high friction between them resulting very large reaction between the substrates. Thus, it is necessary to use larger force to support the substrate than the reaction for executing the fine alignment of the substrates tightly stuck by the very high friction. From the result of measuring the reaction between the 13.3 inches substrates having post spacer, in the atmosphere, it reaches 80-100 kgf. The larger sizes of the substrate and the larger number of the post spacers, the larger reaction is occurred between the substrates, thus the larger supporting force is necessary to perform the fine alignment. It is estimated that the reaction may reach to a few thousands kgfs if a plural of liquid crystal panels is built into the one pair of the substrates larger than 1 square meter each. In such a case of the large force which is loaded on a glass holding apparatus, the rigidity is required to prevent the glass holding apparatus from distortion beyond the tolerance. More specifically, in order to achieve the accuracy of alignment of 3 micrometers, the tolerance of the distortion should be totally 1 micrometer or less in the whole glass holding apparatus. Such an apparatus having the high rigidity is impractical from not only the size but also the cost point of view. And, if the substrate is held by such large force with vacuum chucking, the force directing to tear away the substrates is generated, and thus, it causes that the sealant is broken up to allow the air bubble catch into the cell and/or the glass substrate is broken down.
On the other hand, the inventor found that the fine alignment could be practiced by using a film having high coefficient of static friction. In the method of fine alignment of the subject invention, the film having high coefficient of static friction, such as silicone rubber sheet, is contacted to the upper surface of the upper substrate of the laminated substrates, and then the upper substrate can be moved horizontally by using the friction between the film and the surface of the substrate caused by applying the normal force to the interface between the film and the surface of the substrate. In this situation, the force loaded to the substrates is direct to press the substrates together, rather than to tear away them seen in the vacuum chucking. And, by using the film having high coefficient of static friction, only small force is required to hold and move the substrate, and then, not so high rigidity is required to the glass holding apparatus. It means there is very few risks to break up the sealant or the glass substrate. Particularly, if the film having the coefficient of the friction larger than 1.5, more preferably, larger than 2.0, is used, the normal force of 50-60 kgfs is necessary to cause the friction between the film and the surface of the substrate being larger than the reaction of 100 kgfs. Therefore, it is easy to realize the apparatus and method of the fine alignment.
The inventor also found that the upper substrate is floating on the liquid crystal dropped thereon without contacting the tip of the post spacers soon after the releasing the supporting of the substrate held by mechanical chucking, by the detailed investigation of the process to fill the liquid crystal into the substrates by dropping method. Such the floating situation, the reaction is very small because it depends only on the friction between the liquid crystal and the substrate but not on the large friction between the tip of the post spacers and the substrate, and as the result, it is only a few kgfs for the 13.3 inches substrates. And, after that, it takes a few 100 seconds to touch the tips of the post spacers with the upper substrate in the vacuum, which is sufficiently enough to complete the fine alignment. However, it takes only a few 10 seconds to touch them in the atmosphere, which is not enough to do the fine alignment. Therefore, it is preferred to execute the fine alignment in the vacuum with using the film having high coefficient of friction during a small reaction is existed between the substrates, which is in the floating state of the upper substrate on the liquid crystal.
The present invention was made based on the foregoing findings, and provides a method for stacking a first rectangular sheet and a second rectangular sheet. This method comprises the steps of: arranging the second sheet bent downward in a projecting shape by supporting two opposing sides oppositely to the first sheet; bringing the first sheet and the second sheet close to each other to have a predetermined distance; applying a perpendicular load to the second sheet; and stacking the first sheet and the second sheet by releasing the supporting of the two sides while the perpendicular load is in an applied state.
In the sheet stacking method of the present invention, the perpendicular load is preferably applied to the middle portion of a supporting span in the second sheet. This is advantageous in that since a balance is set with the load applied on the second sheet by supporting, the displacement or shifting of the second sheet can be prevented when the supporting is released.
Furthermore, regarding the releasing of the supporting of the two opposing sides, the simultaneous supporting releasing thereof is preferable for preventing the displacement of the second sheet.
Still furthermore, in the sheet stacking method of the present invention, controlling of the bending amount of the second sheet to a specified amount is preferable for the control of a distance between the first sheet and the second sheet.
The present invention provides a stacking apparatus for executing the foregoing sheet stacking method. Specifically, the sheet stacking apparatus of the present invention is designed for stacking a first rectangular sheet and a second rectangular sheet, and comprises: a stage having a mounting surface for flatly mounting the first sheet; first supporting means adapted to support each of two opposing sides of the second sheet and movable backward and forward in a direction parallel to the mounting surface; and a loading member for pressing the second sheet supported by the first supporting means in a direction orthogonal to the mounting surface.
In the sheet stacking apparatus of the present invention, distance adjusting means is preferably comprised to adjust a distance between the first sheet mounted on the stage and the second sheet supported by the first supporting means.
According to the present invention, the sheet stacking apparatus further comprises second supporting means adapted to support each of two sides orthogonal to the two opposing sides of the second sheet and movable backward and forward in a direction parallel to the mounting surface. This second supporting means is capable of controlling the bending amount of the second sheet by supporting the second sheet in a position lower than the first supporting means by a specified distance.
Further, in the stacking apparatus of the present invention, the first supporting means and the second supporting means are preferably movable backward and forward independently of each other. After supporting by the second supporting means is released, supporting by the first supporting means can be released. Alternatively, a reverse operation like that can be performed.
The present invention can be applied to a manufacturing method of a liquid crystal display panel. Specifically, the method of the present invention is adapted to manufacture a liquid crystal display panel having a pair of glass substrates disposed oppositely to each other with a predetermined distance and secured by sealant formed along the peripheral portion thereof, and liquid crystal sealed in a region inside the sealant between the pair of glass substrates. The manufacturing method comprises the steps of: (a) flatly holding one of the pair of substrates; (b) dropping liquid crystal onto the one substrate; (c) supporting the other of said pair of substrates so as to be bent by supporting two opposing sides thereof, and a bending amount is controlled to be a specified value; (d) bringing the one substrate and the other substrate close to each other to reach a predetermined distance; (e) applying a load in the bending direction of the other substrate with respect to a position having maximum bending of the other substrate and/or a vicinity of the same; and (f) releasing the supporting the other substrate after the application of the load.
In the manufacturing method of the liquid crystal display panel of the present invention, the steps (d), (e) and (f) are preferably carried out in vacuum for the purpose of preventing bubbles from remaining in the liquid crystal display panel.
In addition, in the manufacturing method of the liquid crystal display panel of the present invention, the step (e) is preferably carried out after the alignment of the pair of glass substrates with each other. Of course, the alignment can be executed after executing the step (e). Specifically, it is preferable for the prevention of the displacement that alignment is carried out before and/or after the step (e) of applying a load in the bending direction of the other glass substrate with respect to the position having maximum bending of the other glass substrate and/or the vicinity of the same, and then the step (f) of releasing the supported state of the other glass substrate is carried out after the application of the load.
Further, if necessary, the fine alignment can be executed by applying a normal force to a film having high coefficient of static friction. It is preferred to execute the fine alignment in vacuum, soon after releasing the supported state.
The present invention provides a manufacturing apparatus for realizing the foregoing manufacturing method of the liquid crystal display panel. Specifically, the apparatus of the present invention is adapted to manufacture a liquid crystal display panel having first and second substrates arranged oppositely to each other with a predetermined distance and secured by sealant formed along the peripheral portion thereof in a picture-frame, and liquid crystal sealed in a region inside the sealant between the first and second substrates. The manufacturing apparatus comprises: sealant applying means for applying sealant on the first substrate in a picture-frame; a dispenser for dropping liquid crystal onto the first substrate applied with the sealant; stacking means for stacking the first substrate having the liquid crystal dropped thereon and the second substrate; a vacuum chamber for performing the stacking in vacuum; and sealant curing means for curing the sealant on the stacked first and second substrates. The stacking means includes: a stage having a mounting surface for flatly holding the first substrate; first supporting means adapted to support each of two opposing sides of the second substrate and movable backward and forward in a direction parallel to the mounting surface; loading member for pressing the second substrate supported by the first supporting means in a direction orthogonal to the mounting surface; and distance adjusting means for adjusting a distance between the first substrate mounted on the stage and the second substrate supported by the first supporting means.
Also, the manufacturing apparatus of the liquid crystal display panel of the subject invention comprises a fine alignment device. The fine alignment device comprises a film having high coefficient of static friction disposed directly on the surface of at least one substrate of the pair of substrates, and a loading plate for loading a normal force to the film.